Can confirm in 0.12.2 Windows 10 edition as well, spawners in nether fortress are empty...
Resolution: Invalid?? What does that mean? You will not support us users with Windows account names with special characters? I find that a bit offensive in that case...
I have the same experience in Windows 10 edition.
But I created the world in 0.12 and loaded it again in 0.12, and have this issue.
Yes that works, just wanted you to know that it doesn't handle special characters, as probably a lot of users have this, and it makes Minecraft crash. So maybe you could announce it so they know, guess you will fix it in the next release anyway.
I finally found the real reason, it's because Minecraft Windows 10 Edition can't handle special characters in the user folder name. For example my user folder is C:\Users\Bjørn. So it probably crashes when it tries to access or save settings.
So I guess that's your problem also, Sönke.
I found that if I installed it in a new fresh Windows 10 user account (not upgraded from Windows 8) it works.
I will check tonight.